In hypertension, regardless of its cause, renal vascular resistance increases, shifting the pressure natriuresis curve to the right. The afferent (Af-Art) and efferent arterioles (Ef-Art) account for most renal vascular resistance; they control glomerular filtration rate (GFR) and peritubular pressure, and consequently renal function. Af- and Ef-Art resistances are regulated by a balance between vasopressors (angiotensin, adenosine via the A1 receptor, reactive oxygen species) and vasodepressors (kinins, NO, adenosine via the A2 receptor). The Af-Art is also controlled by tubuloglomerular feedback (TGF). TGF operates via the macula densa, which senses Cl-, Na+ and/or solute load and sends a paracrine signal to the extraglomerular mesangial cells and/or the effectors of TGF, which are the Af-Art and Ef-Art. In addition to the TGF signal itself, the macula densa produces autocrine and paracrine factors that alter TGF either by acting on the macula densa or the Af- and Ef-Art, respectively. NO produced by macula densa neuronal NO synthase (nNOS), which attenuates TGF, is one such autacoid. We have evidence that the Ef-Art dilates rather than constricts during TGF and that the Ef-Art response to vasoactive hormones is modulated by paracrine factors produced by the glomerulus. Thus control of the renal microcirculation is complex and difficult to examine in vivo. For this reason, we propose to use a technique we developed that consists of in vitro perfusion of a microdissected Af- or Ef-Art and adherent tubular segment containing the macula densa. Using this preparation, we hypothesize that different and efferent resistance are regulated by autocrine and paracrine factors released from the glomerulus and the macula densa. Factors that promote dilatation include NO, prostaglandins (PGs) and 5,6 epoxyeicosatrienoic acid (EET). These are counterbalanced by factors that promote vasoconstriction including Ang II, thromboxane, 20- hydroxyeicosatetraenoic acid (HETE) and reactive oxygen species. Paracrine factors such as adenosine may have dilator or constrictor actions depending on whether A1 or A2 receptors are expressed on the target tissue. In Aim 1 we will test the hypothesis that when macula densa nNOS is increased, such as during low salt intake, NO released by the macula densa regulates basal Af-Art resistance even when the NaCl level in the macula densa is very low. In this situation, NO acts not only at the macula densa but also by diffusing from the macula densa to the Af-Art where it causes dilatation, thus preserving renal blood flow despite high renin. In Aim 2 we will test the hypothesis that by quenching NO released by macula densa nNOS, O2- determines a) the magnitude of TGF and b) whether NO acts only in an autocrine manner in the macula densa itself, or also in a paracrine mode by diffusing to the Af-Art and causing dilatation. In Aim 3 we will test the hypothesis that increased intracellular calcium in the macula densa acts as both a positive regulator by contributing to macula densa release of ATP and formation of adenosine in the interstitial space and a negative regulator of TGF by activating nNOS. In Aim 4 we will test the hypothesis that the glomerulus releases paracrine factors that control downstream Ef-Art resistance and consequently its own filtration pressure. In Aim 5 we will test the hypothesis that the mechanism of Ef-Art TGF is similar to Af-Art TGF, save that adenosine acts on the Ef-Art A2 receptor, causing dilatation. Thus reducing NO by inhibiting nNOS in the macula densa will potentiate Ef-Art TGF (greater dilatation).